Recently, in a rapidly-developing mobile communication system (for example, Personal Handyphone System: hereinafter, referred to as “PHS”), for efficient use of radio wave frequencies, SDMA (Space Division Multiple Access) in which a single time slot of a single frequency is spatially divided so that personal stations of a plurality of users can establish space division multiple access to a radio cell station system has been proposed.
In the SDMA, similarly to TDMA (Time Division Multiple Access) that has already been put into practical use, a time period (1 frame) using one frequency is divided into several time slots so as to allow each user to use different time slot. Accordingly, time-position control (burst synchronization control) for accurately transmitting signals to allocated time slots in order to avoid overlapping of the signal with other user's transmission is required at the time of transmission.
In the PHS, synchronous position detection utilizing what is called correlation synchronization has conventionally been known as a method of locating a synchronous position of a signal received from a personal station (hereinafter, referred to as “PS”) in a cell station (hereinafter, referred to as “CS”).
According to the synchronous position detection utilizing what is called correlation synchronization, a time-position at which an autocorrelation value attains a peak is specified as the synchronous position in the following manner. Specifically, in view of the fact that a reception signal from a PS includes in each frame a reference signal (a unique word signal: UW signal) section consisting of an already-known bit string common to all users, a CS calculates a correlation value (autocorrelation value) of an already-known waveform of the reference signal stored in a memory in advance and a waveform section cut out while shifting the waveform of a reception signal from the PS having a prescribed length.
The synchronous position detection utilizing such correlation synchronization, however, is based on the premise that the CS receives a signal from a single PS, and it is not adapted to a case in which the CS receives signals of the same frequency from two or more PSs in an overlapped manner.
In the SDMA in particular, a single time slot of the same frequency is spatially divided and resultant channels are allocated to a plurality of PSs. Therefore, it is highly likely that two or more signals of the same frequency arrive at the CS in an overlapped manner.
If synchronous positions of the reception signals from the PSs of the plurality of users coincide with one another, reference signal sections of the reception signals overlap with one another and the users cannot be identified or separated from one another, resulting in interference among the users.
In order to solve such a problem, control such as forming a directivity pattern in accordance with a direction of the PS by means of an adaptive array apparatus or slightly shifting a signal transmission time for each of the plurality of PSs establishing space division multiple access has been carried out so far in the CS.
In addition, recently, Japanese Patent Laying-Open No. 2001-231072 and “A Study of Training Signal Pattern for SDMA System,” Takeo Miyata et al., Technical Report of IEICE, Vol. RCS2002-47, pp. 177-182, 2002, for example, have proposed a measure to prevent mutual interference among a plurality of users by appropriately separating and extracting communication data of each PS by designating different reference signal for each PS in the CS so that each user uses the designated reference signal.
Here, an operation for controlling allocation of a reference signal performed by the conventional CS on the PS will be described with reference to FIG. 7.
FIG. 7 is a flowchart illustrating an operation performed by a control unit within the CS when a link channel establishment request is received from the PS for initiating a call or data communication.
It is noted that, as the precondition for the operation in FIG. 7, a reference signal is stored as unique word information in advance in a unique word storage unit within the CS in a fixed manner.
The unique word information represents information obtained by associating a unique word value with information indicating a status that the unique word has been allocated. The unique word takes a 16-bit value such as “0011110101000110” expressed as a binary number, or a 32-bit value. In addition, the unique word takes a value specifying a PS such as user 1, user 2 and so on as a value indicating a status that the unique word has been allocated, or a value indicating that the unique word has not yet been allocated.
Referring to FIG. 7, initially, when a link channel establishment request is received from the PS (step S301) or when a link channel reestablishment request is received (step S302), a control unit searches for a channel available for allocation to the PS (step S303).
If there is no channel available for allocation (step S304), control such that a link channel allocation rejection notification is transmitted to the PS is exerted (step S308).
On the other hand, if there is a channel available for allocation (step S304), referring to the unique word held in the unique word storage unit, the control unit determines that an unallocated unique word is to be allocated to the PS and updates the unique word information so as to associate the unique word with the PS (step S305).
In addition, the control unit transmits to the PS a link channel allocation notification including notification of the determined, one unique word (step S306).
In this manner, the conventional CS receives from the plurality of PSs communication data in a form including a unique word different from each other. Therefore, using each unique word as the reference signal, communication data of each PS can appropriately be separated and extracted from the signals that have established space division multiple access by an adaptive array based on minimum mean square error (MMSE).
Accordingly, the CS can communicate with the PS in the SDMA while avoiding interference with other PS and maintaining communication quality.
As described above, in the CS shown in FIG. 7, when an unused unique word among a plurality of unique words held in the unique word storage unit is allocated to the PS that has issued a connection request in a link channel establishment phase, the unique word to be used in subsequent communication is fixed to this allocated unique word.
Meanwhile, in a communication environment in the SDMA, interference tends to be always intense and a spatial correlation value of the users varies every moment depending on increase or decrease in the number of users that establish multiple access to an identical channel. Therefore, if the reference signal including the unique word allocated at the time of connection request is used, synchronous position estimation accuracy and interference canceling performance by the adaptive array may deteriorate due to subsequent change in the number of multiplexed connections.
From the foregoing, an object of the present invention is to provide a radio cell station apparatus capable of estimating with high accuracy a synchronous position of a signal for each user regardless of change in the number of users that establish multiple access as well as separating and extracting a signal of a desired user in a stable manner, a personal station, a method of controlling a reference signal, and a program for controlling a reference signal.